In the conventional copying process using a liquid-type developer, a toner-image-bearing copying sheet or paper is heated from the back side thereof by means of a heat-application roller which is maintained at a high temperature to evaporate a carrier liquid component from the toner images formed on the copying paper, in order to fix toner images on the copying paper. However, the above-mentioned toner image fixing method requires a large amount of thermal energy for fixing toner images on the copying paper. Furthermore, in the case of a duplex copying process, when the aforementioned large amount of thermal energy is applied to a copying sheet for second image fixing on a second side the copying sheet, toner images which have been already fixed on a first side thereof are also heated, so that there is the risk that the already fixed toner images are melted again. The melted toner stains heat-application members, or the images formed on the first side of the copying sheet tend to become blurred.
Therefore, various studies have been made, with a special emphasis laid on the image fixing method in which the surface of a toner-image-bearing sheet is brought into direct contact with the heat-application roller, from the viewpoints of saving energy, increasing copying speed and obtaining high quality images.
However, in the case of commercially available liquid-type developers, both the softening point and the flow-initiating point of the toner components thereof, measured by a capillary rheometer, are generally low and close to each other. The reason for this is that the above commercially available liquid-type developers are designed as to smoothly spread on the copying paper and easily permeate thereinto when fused for image fixing. To surely fix images on the copying paper using the aforementioned commercially available liquid-type developers, the temperature of the employed heat-application roller must be accurately set within a considerably narrow range. When the temperature of the heat-application roller is lower than the above range, image fixing will become imperfect. On the other hand, when it is higher than the above range, a so-called "off-set phenomenon" will occur. Specifically, when the heat-application roller which is heated to a temperature higher than the above-mentioned range is brought into pressure contact with a toner-image-bearing copying sheet, part of the hot, melted toner on the copying sheet is transferred to the surface of the heat-application roller, and the thus transferred toner is disadvantageously re-transferred to the copying paper and stains the same to form the so-called ghost images on the copying sheet.